Color filters employed in liquid crystal displays and the like are each primarily fabricated by applying a coating formulation, which is formed of a photoresist (photosensitive resin solution) and a pigment dispersed in the photoresist, onto a color filter substrate by the spin coating method, a coating method or the electrodeposition method, exposing the thus-formed color coating film to light through a photomask, and then developing the thus-exposed coating film to develop a pattern such that pixels are formed, that is, by a so-called pigment dispersion process (a process making use of a pigment as a colorant).
As a coloring pigment for a coating formulation to be used for the formation of blue pigments, a dioxazine violet pigment of a violet color (C.I. Pigment Violet 23) has been mixed and used for the purpose of color correction (color matching) in addition to ε-phthalocyanine (C.I. Pigment Blue 15:6) as a blue pigment. Especially with a view to improving the blue pixels of color filters for TV monitors, the demand for which is markedly growing in recent years, into pixels of more preferred color quality, there is an outstanding requirement to increase the mixing ratio of the dioxazine violet pigment to ε-phthalocyanine as a blue pigment.
Upon dispersing the above-described mixed pigment of the ε-phthalocyanine pigment and the dioxazine violet pigment in a dispersion medium such as a photoresist, however, the mixed pigment cannot be fully dispersed when it is simply dispersed in a conventional disperser. If blue pixels are formed from the resulting coating formulation, the pixels lack optical transparency and have insufficient light transmittance as blue pixels for color filters. The coating formulation comprised of the above-described mixed pigment is, therefore, dissatisfactory as a blue-pixel-forming coating formulation for color filters.
As resins generally employed in photoresists as dispersion media for pigments, on the hand, acrylic polymers of high acid value are primarily adopted so that color coating films after exposure can be developed by an aqueous alkaline solution. However, a coating formulation comprised of the mixed pigment and a photoresist, which contains such an acrylic resin of high acid value, involves problems in that the pigment tends to undergo aggregation and the viscosity of the coating formulation tends to become higher and also that with time, the coating solution thickens and the storage stability of the coating formulation deteriorates.
Upon formation of pixels of the respective colors of a color filter with coating formulations accompanied by such difficulties as described above, the coating formulations employed are each applied onto a substrate by the spin coating method and the resultant coating film is then exposed to light and developed to form a pattern. When the coating formulations so employed have high viscosity or show thixotropic viscosity due to aggregation of their pigments, the color coating films (before their exposure) formed from the respective coating formulations swell up at central parts. Fabrication of a large-size color-filter, therefore, leads to a problem in that unevenness and a difference occur in color hue and color density, respectively, between pixels on a central part of the substrate and those on a peripheral part of the substrate.
Although each coating formulation for a color filter generally has a pigment concentration in a high concentration range of from 5 to 20 wt %, it is therefore necessary that its dispersion state is free from aggregation of pigment particles, its viscosity is low (for example, 5 to 20 mPa·s or so) compared with general room-temperature drying coating formulations or baking coating formulations, and it is excellent in storage stability.
To meet the above-described requirements, methods have been proposed to date for cases where the pigment is ε-phthalocyanine (C.I. Pigment Blue 15:6), including the method that adds a substituted derivative of phthalocyanine blue or a substituted derivative of dioxazine violet as a dispersant to the pigment and the method that treats the pigment with the above-described derivative (see, for example, Patent Documents 1 to 6).    Patent Document 1: JP-A-56-167762    Patent Document 2: JP-B-1-34268    Patent Document 3: JP-A-4-246469    Patent Document 4: JP-A-6-240161    Patent Document 5: JP-A-6-240162    Patent Document 6: JP-A-7-188576